1. Field of the Invention
This invention relates to the field of ionizing radiation detection. More particularly, the invention relates to scintillation detectors of the type that are typically coupled to a photomultiplier tube and are used for radiation monitoring such as clinical measurement of isotopes. The invention specifically relates to well-type scintillation counters having a high efficiency for measuring the degradation rate of radiopharmaceutical solutions and resolving the distribution of energy emanating from the radioactive source.
2. Description of the Prior Art
Radiation detection devices generally rely upon the property of substances called scintillation phosphors which, when irradiated with x-rays or gamma rays, are capable of converting the energy into light flashes. Each flash is referred to as an event.
The phosphor is coupled, optically, to a device such as a photomultiplier tube which converts the light flash or scintillation to an electrical impulse, and which amplifies the same. The amplified current output from the tube is sent to electronic devices which display the information in usable form. Through the scintillation counter has become a familiar instrument for the detection of many types of ionizing radiation in a wide variety of applications, more sophisticated utilization of scintillation phosphors has demanded an increasing attention to the improvements in quantifying the degradation rates of radioactive sources and simultaneously improving the resolution of energy distribution of the incident radiation.
Improvements in the counting efficiency of the source, that is, the ability to record the number of events per unit time detected by the phosphor as compared to the degradations suffered by the source which results in the emanation of an x-ray or gamma ray, has been achieved by machining a blind hole or well in a phosphor, typically a circular hole in a right cylinder, and placing the radioactive material as close to the bottom of such a hole as the surrounding housing permits. The ideal situation would be to place th source in the center of a sphere, thereby insuring that the radiation which is emanated isotropically will always interact with the phosphor material. The machining of holes in a solid phosphor departs from idealism and disturbs the geometrical aspects associated with the collection of light uniformly at any given surface to which the photomultiplier may be coupled.
Prior attempts to equalize light collection output in detector assemblies have involved polishing various portions of the crystal while roughening other portions; using reflective or absorptive coatings or paint upon the surface of the protective housing in which the crystal is placed; the use of packing powders of magnesium oxide or aluminum oxide having specified reflective properties, around the crystal, etc. These efforts have all involved time-consuming additional steps in the assembly and have contributed considerably to the cost of the same. In addition, reproducability from one crystal to another has been difficult to obtain, and within each crystal, efforts to closely regulate the optical properties at various locations within the crystal assembly have been frustrating.
One such prior art method of altering the surface reflectivity of a scintillation crystal assembly is described in U.S. Pat. No. 3,102,955. This patent attempts, with some success, to improve the uniformity of scintillation light output by compensating the reflector system by polishing. This patent does not deal with well-type assemblies having a blind hole in the crystal and attempts to use similar techniques with well assemblies have been expensive and have met with marginal success.